Recently, for the sake of environmental measures, weight reduction and downsizing of various devices with better characteristics of steel materials have been required. For example, as an eco-friendly approach in the field of automobiles, there have been growing contradictory demands that exhaust gas should be reduced by improving fuel efficiency through weight reduction, safety should be secured by increasing strength against a collision, and costs should be reduced. As one solution to these, characteristics improvement including increasing the tensile strength of steel materials is an important subject. When electromagnetic steel sheets as a functional material are used as various devices, the issues of weight reduction and downsizing are inseparable. To solve these problems, improvement in electromagnetic characteristics is essential for the electromagnetic steel sheets.
As an example of the methods for improving the characteristics of a steel sheet, there is characteristics improvement by batch annealing. For example, in order to address the trouble of stretcher strain that can occur when cold-rolled steel sheets, which are generally used for automobiles and household electric appliances, are formed and a fluting phenomenon that can occur when cans are formed, these phenomena can be avoided by annealing and temper rolling.
The temper rolling and subsequent strain aging can vary depending on how annealing is performed. In other words, objectives differ according to the selection of batch annealing or continuous annealing. Because the batch annealing can take long heating and soaking times, carbon (C), nitrogen (N), and the like that are dissolved are easy to be precipitated. As a result, the batch annealing can obtain a steel sheet that is easy to be softened and has a characteristic of small aging effect. The continuous annealing works in reverse.
The batch annealing plays an extremely important role in the electromagnetic steel sheet. That is, for the electromagnetic steel sheet, annealing by a batch annealing furnace can achieve, not only precipitation of dissolved elements, but also characteristics of the electromagnetic steel sheet as the original purpose by performing recrystallization. In other words, for the electromagnetic steel sheet (that is cylindrically wound to be formed in a coil shape), annealing by the batch annealing furnace is an essential manufacturing process that cannot be omitted or replaced with any other processes.
However, a coil obtained by annealing contains some defects (defects such as “edge elongation” in the upper part of a coil, “edge distortion” in the lower part of a coil, “center elongation and longitudinal wrinkles” in the central part of a coil, and characteristics degradation such as inability to improve characteristics involving specific phase transformation). Given this situation, in order to use the defective coils as steel materials, as for shape defects, by passing the coils through a defect detection system or a tension leveler in a recoiling line, the coils are made usable as products with defects extracted, defective parts removed, and shapes corrected. Given these circumstances, coils obtained by annealing have had the problems of a decrease in yield before being made into products, a decrease in production efficiency, and high costs associated with inspection and shape correction.
When the coil obtained by annealing does not have characteristics as good as or better than predetermined characteristics in terms of the characteristics improvement, the coil is used with cutting off a deteriorated part. For this purpose, the coil has to be passed through an inspection line, marking and online cutting-off have to be performed, and the coil has to be wound again. This causes decreases in product pass rate and production efficiency. Because the coil is passed through the line again and is wound while performing characteristics measurement thereon, the cost for performing the measurement is added, leading to a significantly large cost increase.
Given these circumstances, the following various measures have been developed for such various problems in the batch annealing furnace. By performing these measures, the occurrence of defects after performing the measures can be reduced as compared with conventional cases.
For example, in a technology disclosed in Patent Literature 1, defects occurring inside a coil are observed and measures are carried out on the defects. In other words, the technology disclosed in Patent Literature 1, in order to reduce defects occurring in the lower part of the outer periphery of a coil, welds coils having different sheet thickness and performs recoiling so that a thicker sheet thickness is positioned on the outer side and a thinner plate thickness is positioned on the inner side, thereby forming one coil and performing annealing thereon.
A technology disclosed in Patent Literature 2, in order to resolve the sticking and loosening of a steel sheet as a coil, attempts to prevent the sticking and loosening by managing a temperature difference at cooling.
A technology disclosed in Patent Literature 3 refers to that the problem of seizure flaws can be resolved by making the structure of a batch annealing furnace a double structure equipped with an inner cover and setting a temperature condition of cooling speed to 5.0 to 15.0° C./Hr.
Patent Literature 4 discloses a method that, without managing the heating and cooling of a furnace in terms of speed, determines the relation between a critical stress at which seizure occurs at annealing and temperature in the radial direction, and based thereon, avoids flaws.
Patent Literature 5 and Patent Literature 6 disclose coil defects occurring at annealing in an annealing furnace and measures against the defects. For example, Patent Literature 5 discloses a method that prevents buckling in a coil by performing covering inside the coil. Patent Literature 6 discloses that defects occurring in a coil are resolved by forming a uniform temperature distribution within a furnace. Relating to this, the technology disclosed in Patent Literature 6 performs heating so as to give the uniform temperature distribution by covering or lining an inner cover of the furnace with a heat insulating material.
A technology disclosed in Patent Literature 7 forms a concave recess at the central part of an inner cover of a furnace and performs heating with this recess also from inside the coil at heating, thereby making a temperature distribution inside the coil uniform. The technology disclosed in Patent Literature 7 makes the temperature distribution within the coil uniform also at cooling by a similar effect. The technology disclosed in Patent Literature 7 discloses a method that can thereby reduce a stress occurring within the coil and reduce defects, and at the same time, reduce heating and cooling times and improve productivity.
Patent Literature 8 discloses a technology that puts a device that can perform heating and cooling of a coil into a furnace and heats and cools the inner and outer surfaces of the coil directly, thereby making a temperature within the coil uniform and improving productivity as well as a reduction in defects.